Electrical power generation and conversion systems convert mechanical power supplied by a prime mover, such as a gas turbine or internal combustion engine, to direct current (DC) or alternating current (AC) electrical power. Typically, the prime mover drives a wound field synchronous generator. Wound field synchronous generators are desirable because it is easy to regulate their output by way of excitation current control. In the case of AC power generation, the AC frequency of the generated power will be proportional to the rotational velocity of the wound field synchronous generator. This is satisfactory for variable speed variable frequency (VSVF) power systems. Variable speed constant frequency (VSCF) systems and DC power systems may employ a downstream power converter with such a generator.
To improve power density of components used in such power generation and conversion systems, it is necessary to establish a relatively high fundamental frequency of operation. To improve efficiency and reduce thermal management, it is desirable to minimise power converter switching losses.
The main disadvantage of the wound field synchronous generator of the conventional type is its sliding rings and brushes for its rotor. The main disadvantage of the wound synchronous generator of the brushless type is its complex rotor structure.
However, it is possible to employ a synchronous generator of the permanent magnet (PM) type. The main disadvantage of PM generators is voltage regulation. Although it is possible to overcome this disadvantage with special designs that add a field winding for voltage control purposes. The extra field winding may be in the rotor, in which case it has the same disadvantage of sliding rings and brushes as a wound field synchronous generator has. The extra field winding may alternatively be in the stator, in which case the winding either magnifies or reduces the rotor PM magnetic flux or diverts the main magnetic flux in the stator core. In all cases, the additional field winding contributes to an increase in the volume and weight of the PM generator.
It is also possible to employ a generator of the induction type. The main disadvantage of self-excited induction generators are large running capacitors and poor voltage regulation or solid-state converters that must provide large excitation power that exceeds twenty five percent of the generator rating.